The identification of crystallographic microstructures is of considerable importance in many fields of technology, for example in the case of metallic workpieces which are subjected to high stress, such as in aeroplanes and automobiles.
As a rule, the first thing is to find out about the chemical composition of the sample to be analyzed. A standard method of material analysis, which is used for these purposes, is energy-dispersive X-ray spectroscopy (EDX). An electron beam with uniform energy is directed to the relevant measurement point of the sample and the resulting X-ray emission is detected. The detected characteristic X-ray radiation reveals the elementary composition of the sample.
Electron Backscatter Diffraction (EBSD) is a method of structural analysis which serves to identify crystals in a sample. In this method, the diffraction of electrons on the crystal lattice (the so-called diffraction image) is evaluated for the purposes of phase analysis or crystal structure analysis. A diffraction image consists of a series of diffraction bands, the position of which depends on the crystal structure at the specific location within the sample and from the local orientation of the crystal. The evaluation of the diffraction images therefore necessarily requires knowledge of the prevailing crystal structure. The knowledge about this crystal structure is used to predict how the diffraction bands should be positioned in the diffraction image for a given orientation.
The structure data of many thousand known crystal structures, which are required to predict the diffraction bands, are compiled in databases and serve as references for identifying previously unknown phases of a measured sample. Based on said structure data, mathematical methods can be used to predict the position of the bands of interest within the patterns of a plurality of orientations. In practice, certain selection criteria, mostly relating to the chemical composition, are applied to pre-select crystal structures in the database which are expected to be present in the sample.
After the diffraction image of the sample has been obtained, the orientation is determined where the prediction is most similar to the measurement, based on the diffraction image of the unknown sample and by means of suitable search methods. If the selected crystal structure is correct, a good similarity is obtained; otherwise the similarity is poor. Examples of methods for identifying the orientation are, for example, described in the publications: Wright, S. I. and B. L. Adams: Automated Lattice Orientation Determination from Electron Backscatter Kikuchi Diffraction Patterns, Textures and Microstructures, vol. 14, pp. 273-278, 1991. doi: 10.1155/TSM.14-18.273; Schwarzer, Robert A.: Automated Crystal Lattice Orientation Mapping Using a Computer-controlled SEM, Micron, Volume 28, Number 3, June 1997, pp. 249-265(17); or Zaefferer S. and R. A. Schwarzer: On-line Interpretation of Spot and Kikuchi Patterns, Materials Science Forum Volumes 157-162 (1994) pp. 247-250.
The degree of similarity is primarily indicated by the number of diffraction bands which are successfully explained within a certain tolerance. This means a certain difference between the theoretical diffraction bands and those actually measured is accepted. The degree of this difference—the angular error—is a secondary indicator of similarity. If a sample is expected to contain several crystal structures, the evaluation procedure is repeated for all candidates. The candidate with the best similarity is identified as belonging to the specific location within the sample.